There's a great book called 'Sun of York', which describes a young nobleman from an impoverished family and his struggle in the WOTR. The only armour he had was an extremely elderly set that belonged to his great-grandfather. He updates it piece-by-piece in the story by looting his defeated opponents.
It's easy to imagine many men fighting with obsolete kit, especially if there are several sons on campaign. A suit of armour was a massive investment and only the richest could afford the latest designs. I guess you could compare it to cars on the road - a couple are this year's model, most are from the last decade or two and then you will see a few old bangers still limping along from ages back.
Of course, even an old suit of armour is still serviceable if it kept in good condition and a damn sight better than no armour at all.